This invention relates to a secure digital speech communication system, and more particularly to waveform coding with digital samples.
Known digital voice communication systems described in U.S. patents include U.S. Pat. No. 3,349,180 to Coker, which discloses a system for compressing or reducing the amount of bandwidth employed in the transmission of speech information. It involves the transmission of speech information in terms of narrow bandwidth control signals representative of the frequency locations of selected peaks in the speech amplitude spectrum. Kelly et al in U.S. Pat. No. 3,493,684 disclose a vocoder of the narrow band type in which short-time analysis is performed on a speech wave to determine its significant characteristics. Manley et al in U.S. Pat. No. 4,310,721 disclose a system in which analog speech is sampled, converted digitally, and then fed into a fast Fourier transform processor. Croisier et al in U.S. Pat. No. 4,142,071 teach quantizing in a channel bandwith conserving arrangement, and Guanella in U.S. Pat. No. 3,536,833 discloses secret pulse-code speech transmission.
High quality secure voice systems are, and will continue to be, an important part of effective military communication. Achieving high quality is important for voice recognition, interoperability among user communities, and user acceptance of a secure voice system. User acceptance of secure voice equipment is of paramount importance in maintaining the overall security in a communications system. Acceptance or rejection of a system is primarily based on factors such as intelligibility, voice recognition, and robustness with respect to channel errors and background noise. These are the properties which are present in "high quality" voice algorithms. On the other hand, achieving reasonably low bit rates is important for reducing bandwidth requirements. The advantages gained from narrower bandwidths include lower transmission costs and the ability to incorporate error correcting or antijamming facilities. Achieving high quality at reasonable low bit rates (9.6-16 kilobits per second) requires an increase in terminal complexity. However, the rapid advances in Large Scale Integration technology will allow more complex algorithms to be implemented on a single chip, thereby keeping costs low.
There are two broad classes of coders used to encode speech digitally: waveform coders and parametric coders. Waveform coders are concerned with preserving the wave shape of the speech signal; therefore, they are somewhat signal independent. This implies that they tend to be robust for a wide range of speaker characteristics and noise backgrounds. Waveform coders usually operate at a medium to high transmission rate in order to achieve good quality. By contrast, parametric coders use a priori knowledge about the source of the signal in order to reproduce the waveform. By adjusting these parameters as the sgnal changes, it can reproduce the speech signal. Parametric coders can operate at rather low transmission rates but tend to be speaker dependent and also lead to unnatural speech quality.